"could have been stated in just this way: %fields = { id => 0, name => 1, age => 2};"

Assign a key/value pair to a hash:

%hash = ( key => "value" );
[download]

Assign a hashref as a hash key with no value:

%hash = { key => "value" };
[download]

perl -MData::Dumper -wE 'my %fields = { id => 0, name => 1, age => 2};
+ say Dumper \%fields;'
Reference found where even-sized list expected at -e line 1.
$VAR1 = {
          'HASH(0x7f9d5a804850)' => undef
        };
[download]

Just sayin'.